Conventionally, in an information processing device such as a PC (Personal Computer), it has been suggested to reduce power consumption by deactivating an external memory and other components provided outside of the CPU.
Here, inside the CPU provided in the PC, provided is a high-speed and small-capacity memory referred to as cache. Meanwhile, outside the CPU, provided is a low-speed and large-capacity external memory such as DRAM (Dynamic Random Access Memory).
The CPU appropriately selects and uses the cache or the external memory to perform operations. For example, the CPU performs a so-called write back in which data are usually written into the cache only, and the contents of the data written into the cache are reflected to the external memory, if necessary.
As an example of an information processing device including a CPU performing such write back, suggested has been an information processing device which can reduce power consumption in such a manner that a necessary program during a resume is previously loaded in a cache, and when it returns to its previous state, checking that the external memory becomes available is performed not by outside hardware but by the program written in the cache (see patent document 1).
Similarly, suggested has been an information processing device which realize a resume function in such a manner that an internal state of a dynamic device is saved to a static device before the power is turned off, and the saved internal state is restored when the power is turned on again (see patent document 2).
[Patent Document 1]
Japanese Laid-Open Patent Application No. 272347/1999 (Tokukaihei 11-272347; published on Oct. 8, 1999)
[Patent Document 2]
Japanese Laid-Open Patent Application No. 230845/1994 (Tokukaihei 6-230845; published on Aug. 19, 1994)
However, the conventional information processing devices stop power supply to the external memory and other components during a resume for reducing power consumption; however, when the CPU performs operations, the need for access to the external memory comes about. In this connection, there are the problems that when they are switched from a resume state to a normal state, it is necessary to activate the external memory again, so that speedy processing is impossible, and power supply to the external memory is stopped only during a resume, resulting in impossibility of sufficient reduction in power consumption.
That is, in the conventional information processing devices, a cache is used for the purpose of saving data for return to a previous state when they are switched to a resume mode, and of checking whether or not the external memory becomes available when it returns to its normal state. Therefore, both of the information processing devices start operations such as processing a task, after the dynamic device and the external memory are returned their previous states. This means that the external memory is always activated except during a resume, so that it is impossible to reduce power consumption sufficiently.